I know this was ages ago, but I think I can offer an explanation.Originally Posted by IRserge
You sleep in cycles of ~90minutes each, if you wake up in the middle of a cycle it is effectively "wasted". Now then, say you go to bed at 11PM and would normally wake at 7.30AM. This would be 8.5hrs which is 5 full cycles(7.5hrs) + a "wasted" hour. Nw then, if you wake up when you are in a deep sleep you will wake up feeling very groggy for an hr or so, if you wake in accordence to your sleep cycles(ie have 7.5 or 9hrs of sleep) then you will feel les groggy(I actually feel very energetic). If then you woke up @ 6.30 to do the cardio it could explain it, as that would be 7.5hrs of sleep.
Did that all make sense?